As for such an optical module including a plurality of light output units, it is a known technique that an isolator is arranged on each of individual light paths of laser diodes (hereinafter, LD) of the respective light output units. For example, in “optical transmission/reception module” in Patent Literature 1 discloses an example of an optical transmission and reception module, and in this conventional module, an isolator is arranged on a transmission light path (an LD light path) instead of arranging it on a reception light path. While there has been no optical module that is similar to this conventional module and includes a plurality of LDs with different wavelengths, as a usual idea, it is conceivable to mount an isolator individually on a plurality of LD light paths. That is, in an optical module including a plurality of light output units, an isolator is located individually on respective light paths that reach a common outgoing port (an optical fiber) from respective LDs. Particularly in an optical module with this configuration, because each of the LDs has a different wavelength to one another, loss may occur to some parts of isolators unless these isolators are individually optimized.
The isolator is explained. Normally in an LD, an outgoing light having a strong one-direction polarization property, which is known as TE (Transverse Electric) polarization (polarization having an electric field vector parallel to an active layer), is stably generated. However, there is a case where operations and properties of the LD become unstable when there is return light to the LD regardless of its polarization direction. Therefore, in order to stabilize operations and properties of the LD, an optical element referred to as “isolator” that blocks reflected return light to the LD while not blocking outgoing light from the laser diode is located between the LD and a portion that can be a reflected portion in a light path (normally, located between a fiber-incident end face and a laser diode). With this configuration, operations and properties of an optical module are stabilized.